1. Field of the Invention
This invention relates to a tubular structure for conveying recirculated engine exhaust gas from a source of gas, such as an exhaust manifold, to a control element, such as an EGR valve.
2. Discussion of Prior Art
Exhaust gas recirculation ("EGR") systems have been in use for many years. The purpose of such systems is to convey a small portion of the exhaust gas leaving the engine to the inlet manifold. The recirculated exhaust gas aids in the control of oxides of nitrogen, and also helps to reduce the octane requirement of the engine.
EGR supply tubes form an integral part of many EGR systems. The purpose of the supply tube is to bring recirculated exhaust gas from a source of exhaust gas, such as an exhaust manifold, to the EGR control element, such as an EGR valve. Such an EGR tube operates in a difficult environment because the tube may be surrounded initially by sub-zero air. During normal engine operation, the tube may be heated to 1200.degree. F. or even higher by exhaust gases passing therethrough. Moreover, this heating process, followed by cooling, is repeated every time the engine is shut down and allowed to cool to ambient temperature. Thus, the thermal stress imposed upon the EGR supply tube is substantial.
The large temperature excursions imposed on EGR tubes have caused designers to construct these tubes with bellows-like sections to accommodate the pronounced, thermally-induced, axial growth experienced with such tubes. Such bellows structures have not generally proven to be satisfactory because the high temperature of the exhaust gases moving through the EGR tube, coupled with the agitation provided by the interior surface of the convoluted walls of the bellows, have caused considerable heating of the bellows structure. This has proved to be unfortunate because stainless steels have generally been necessary to avoid corrosion resulting from hostile exhaust gases, and high temperature grain migration of the nickel portion of the stainless steel has caused embrittlement and subsequent failure of the bellows structure. In this regard, it is well to keep in mind that the environment that the EGR tube encounters is further rendered hostile by the high vibration forces associated with components bolted to internal combustion engines.
U.S. Pat. No. 4,171,689 to Eheim, and U.S. Pat. No. 4,969,445 to Hertweck et al, disclose bellows structures for handling exhaust gases. Neither of the structures disclosed in the '689 and '445 patents obviate the problems associated with prior art EGR supply tube bellows devices.
U.S. Pat. No. 5,158,061 to Monteith, which is assigned to the assignee of the present invention, while solving problems with axial growth of EGR tubes, does not provide flexibility in the bellows joint. Unfortunately, such flexibility is often needed to permit installation of the EGR tube at either the engine plant where the engine is assembled, or at the assembly plant, where the engine is installed in a vehicle.
It is an object of the present invention to provide an EGR supply tube which accommodates both axial growth of the tube due to heating and rotation of one part of the joint with respect to the other, without the problems associated with prior art bellows constructions.
It is an advantage of the present invention that an EGR tube according to this invention is robust in terms of its ability to withstand the adverse environment to which it must operate.
Other objects, features and advantages of the present invention will be apparent to the reader of this specification.